Mirroring by means of a Redundant Array of Inexpensive Disks (RAID) 1 is a technique of improving reliability of data to be written to a storage device.
In mirroring, the same data is simultaneously written to a plurality of devices.
Recently, hard disc drives are beginning to be replaced with solid-state drives (SSDs), which use nonvolatile memory as a storage medium. NAND flash memories are often used as such nonvolatile memories.
A NAND flash memory writes and reads data per page unit, and erases data per block unit, including a plurality of pages. A NAND flash memory is limited as to the number of times of rewriting.
When SSDs comprising NAND flash memories as storage for mirroring are used, since data with the same content will be written thereto, the lifespan will expire approximately at the same time. In some products, since data cannot be read after the lifespan expires, the reliability of data will be decreased.
It is therefore desired to provide difference in the lifespan of storage devices when mirroring is performed using storage devices configured to write and read data per page unit and erase data per block unit, including a plurality of pages.